Digital correction module for video projector

ABSTRACT

Digital correction module for video projector where a correction module is provided to give a non-linear re-imaging of an image so that it is displayed correct on a non-ideal screen (curved screen) where a correction module is provided in each of a plurality of video projectors after a common image source in order to create a large, of several composite sub image, projected image of said image source. The correction module comprises a sequencer ( 1 ), a pixel write control ( 2 ), a parameter storage ( 7 ), a read address generator ( 13 ), a coefficient generator ( 5 ), a scaling generator ( 6 ), an adder ( 8 ), a pixel storage ( 3 ), a vertical mixer ( 9 ), a horizontal mixer ( 10 ) and a look up function means ( 12 ) for correction of differences in the border area of the image of the respective projectors such as geometry correction, soft transition tuning, vignette correction and gamma correction.

[0001] The present invention regards a correction module for a videoprojector, of the type described in the introduction to claim 1.

[0002] The display of images on large screens, particularly curvedscreens, is today used in many areas with varying degrees of success.One of the problems with using several video projectors in order tocreate a large image is that differences arise in the boundary areas ofthe images of the respective projectors.

[0003] Among other things, large screen displays are used in simulators,e.g. for driving practice or within the entertainment industry. Lately,the prices of such systems have been greatly reduced, especially for thesimulator system, which runs underlying physical models upon simulation,as well as instructor and/or student user interfaces. This costreduction has been brought about as a result of the recent developmentsin PC technology.

[0004] Lately, the use of real-time 3-dimensional graphics in theentertainment industry has resulted in a greater volume of sales forsuch systems, which again has resulted in a great reduction in the priceof image generators, in the order of from hundreds of thousands ofdollars to a couple of thousand dollars.

[0005] This price reduction has made possible the purchase of moresimulators, in order to allow a greater number of persons to undertakesimulator training in areas where simulator training has beenextensively used, such as in the military and aviation in general.

[0006] Thus, up until now, the greatest cost associated with simulatorshas been the projection system. New production technology results incheaper projectors, however these are not of a type that canautomatically be used in simulators.

[0007] In this connection, it should be noted that the projection scenefor a simulator is typically constructed as front projection systemsusing one or more projectors (in some cases more than 10) to create apanorama image. In order to achieve a panorama image, it is necessary toproject the image onto a curved screed by using several projectorsarranged side by side and/or possibly on top of each other. The abovecheap projectors are designed to display a single image on a flatscreen, and are therefore not automatically suitable for display on acurved screen.

[0008] The effect of overlapping zones between the various projectionimages is critical for certain simulator applications, it beingnecessary to ensure a seamless transition from one image (channel) to anadjacent image. When viewing a multi-projector image, it is alsoimportant to control the colour and intensity between the projectors inorder to be able to compensate for varying intensities in the image.

[0009] A further problem may be the fact that the standard lens of theabove projector is designed for display on a flat screen. Thus therewill be a limit to how curved the screen may be before the loss ofoptical focus presents a problem.

[0010] The known and expensive technology makes use of CRT (cathode raytube) projectors, which are expensive to purchase and which requireconstant re-calibration. This re-calibration takes place so often thatit is necessary to have extra personnel present during the use of thesimulator to perform frequent calibrations, which also makes the runningof the simulator more expensive. One advantage of this known technologyis the fact that there is no fixed pixel raster for CRT, i.e. thegeometry may be compensated for within reason.

[0011] The new projector technology includes LCD (Liquid CrystalDisplay) and DMD (Digital Micromirror Device), which differ fromconventional CRT (Catode Ray Tube) based projectors in that they arecheaper to buy and have a fixed pixel raster. The advantage of the fixedpixel raster is that it does not drift, thus making continuousrealignment, as in the case of the CRT, unnecessary. One disadvantagehowever, is that the fixed pixel raster makes it impossible tocompensate for the curved screen geometry. When using several CRTprojectors, the images are easily distorted in order for them to appearseemingly correct on a curved screen.

[0012] None of the technologies mentioned have a built-in capability forgiving a soft transition from one image to the next.

[0013] An ideal projector would compensate for all of the above effects.The main requirement however, would be to be able to generate thenecessary geometry distortion, to be able to modulate the intensity(digital colour modulation) for generating soft transitions from oneimage to the next, and compensate for a varying intensity across theimage field. One of the aims of the present invention is to provide theabove.

[0014] In order to avoid operational problems connected with analogueelectronics, the correction must be performed digitally. This means thatthe correction must be carried out at a point where pixel data isavailable in a digital form, i.e. either in the field oscillator or inthe actual projector.

[0015] The above is effected by a correction module of the typementioned at the beginning, the characteristic features of which appearfrom claim 1. Further characteristics of the invention appear from theremaining, dependent claims.

[0016] The correction module according to the invention may be installedas a plug-in module in existing projectors. The only requirement thatmust be fulfilled is that digital pixel data is available in real-time,so that the data stream may be retrieved and re-formatted by thecorrection module. Physically, the correction module can be designed asa PCB-board to be mounted on top of an existing printed circuit board inthe projector.

[0017] The invention may, in addition to being used for projectors in asimulator, also be used in the entertainment business and similar, withone of several possible applications being described in greater detailin the following.

[0018] A simulator projection theatre may be seen as a special case of avideo wall, i.e. a multi-projector system used in a simulator. Analyseshave shown that the construction of a high quality video wall requiresapproximately the same functions as for application in a simulator, themain difference being that a simulator requires a curved screen.

[0019] Important features of a video wall are that it is simple to erectand install, and that there are no analogue operational problems. A softtransition from one image to the next, combined with removal of hotspots, will result in a large, high quality image, as the seams betweenthe projections may be made virtually invisible.

[0020] The geometry correction will accelerate the setting up of theprojector system, because the requirement for exact mechanical alignmenthas been reduced. Digital keystone correction allows great freedom withrespect to the choice of projector/screen within the limits of opticalfocus.

[0021] Setting up the video wall, including the geometry alignment, ismade simpler by use of simple control, e.g. via a PC, laptop, thataddresses all or individual projectors. This will also allowconfiguration data to be backed up on a disk.

[0022] Typical video wall applications require complex video splittersfor an accurate deduction of individual frames from the source signal.One advantage of the geometry correction circuit is that it can be usedto deduct frames directly from the source signal, thus avoiding the needfor an external splitter. Instead, a simple video buffer system or achained structure is used to distribute the source signal to all of theprojectors.

[0023] The system setting will set up all the projectors to derive theirrelevant frames including an overlap zone for blending.

[0024] In the following, the invention will be described in greaterdetail with reference to the drawings, in which:

[0025]FIG. 1 shows a block diagram of the function of the correctionmodule;

[0026]FIG. 2 schematically shows a pixel element in the input image andits transfer to the output image;

[0027]FIG. 3 schematically shows an example of projectors connected toan image source;

[0028]FIG. 4 shows a possible embodiment of the mixer stages in FIG. 2in a block diagram; and

[0029]FIG. 5 shows a gamma correction curve.

[0030] The correction module according to the invention is to performcorrection functions that are required in order to use a projector witha fixed raster for multi-channel projection on a non-flat screen. Theterm multi-channel is to be taken to mean the display of a plurality offrames, with each frame coming from a separate projector.

[0031] Projectors utilising fixed rasters, such as DMD and LCDprojectors, have no possibilities of distorting geometry by usingconvergence control, the way CRT projectors can.

[0032] A further requirement of multi-channel projector systems is thepossibility of carrying out soft transition tuning and vignettecorrection in order to minimise variations in intensity within a channeland make the transition from one image to the next “seamless”.

[0033] Furthermore, the correction module may also be used for smoothre-sampling of images having a different resolution from the outputraster, for instance re-sampling of an SVGA (800×600) or SXGA(1280×10²⁴) image to an XGA (1024×768) high quality image without lossof lines or pixels.

[0034] Before taking a closer look at the construction of the actualcorrection module, its primary functions, i.e. geometry correction, softtransition tuning, vignette correction and gamma correction, will bedescribed in greater detail:

[0035] Geometry Correction:

[0036] This correction is to allow non-linear re-imaging of the image inorder to ensure that it appears correctly on a non-linear screen (curvedscreen or screen arranged at an angle). The user or control software candefine any non-linear projection from an input pixel grid to an outputpixel grid including trapezoidal distortion and non-linear distortionsfor curved screens.

[0037] The projection can be carried out by use of a network of controlpoints (control network) placed no more than 4×4 pixels apart, whichdefines the projection to the new grid. Between the control points, themodules will perform a linear interpolation in order to derive the pixelco-ordinates for each pixel.

[0038] The accuracy of the pixel co-ordinates at the network points andbetween (interpolated) positions must (an absolute requirement) be equalto or better than ⅛ pixel.

[0039] In the projection process (re-sampling), the system must(absolute requirement) use bi-linear interpolation between the fourclosest pixels in the input grid when the calculation of the value foran output pixel is taking place. It is an absolute requirement that theinterpolation be carried out with an accuracy of ⅛ pixel for theinterpolation factors.

[0040] However there are practical limits to how much an image may bedistorted. The limit of distortion is measured based on how much a pixelcan be displaced on the screen 3 relative to its initial position. Themaximum possible displacement is indicated in FIG. 1 by means of arectangle 2 on the screen 3 around the initial pixel position.

[0041] The maximum allowable local and global displacement in thevertical direction is limited by the amount of pixel storage required tostore a partial image for subsequent display. The local displacement inthe horizontal direction is limited by how quickly pixels can be clockedout of the pixel storage unit and processed (if the image is compressed,the pixels must be clocked out and processed at a higher rate than theinitial pixel clocking rate.)

[0042] The bi-linear sampling selected limits the quality of there-sampled image if a scale-up or scale-down by a factor greater than 2is performed.

[0043] Table I shows the limits in the vertical direction given by thesize of the pixel storage unit (128 or 256 lines, however theseparameters must not be considered absolute, as further practicalexperiments may result in further limits.) TABLE I Parameters Value Max.horizontal displace- Limited by pixel clock, i.e. magnification mentMax. vertical displace- 255/511 lines ment Min. horizontal magnifi- 5×local ₍₁₎; 0.6-0.8× total for full quality ₍₂₎ cation Max. horizontalmagnifi- 2.0× cation Min. vertical magnifi- 0.5× local; 255/511 linestotal compression ₍₃₎ cation Max. vertical magnifi- 2.0× local; 255/511lines total expansion ₍₄₎ cation Max. vertical non-linear 255/511 lines₍₅₎

[0044] Soft Transition Tuning:

[0045] The tuning function for a soft transition should preferably beperformed prior to the geometry correction, and will use a similarnetwork of control points in order to provide individual scaling of theinput pixels as a function of the screen position. Each colour component(R, G and B) is scaled separately. The scale factor is to be definedwith at least a 9-bit resolution. For soft transition tuning, thecontrol network must have control points spaced apart by a distance ofno more than 4 pixels in the horizontal direction and up to 4 pixels inthe vertical direction.

[0046] Vignette Correction:

[0047] The vignette correction is carried out at the same stage as thesoft transition tuning correction, using the same circuit. An importantfeature of this correction is that it will compensate for thedifferences in intensity as a function of the position in the projectedimage, and it will compensate for variations in the perceived intensitycaused by pixels covering different screen areas in the projectortheatre.

[0048] The vignette correction is to be carried out with an accuracy ofat least 9 bits in the calculation. This correction is to be allowed toscale the pixel values down by at least 25% relative to the fullintensity. The control points for the vignette correction are to bespaced no more than 16×16 pixels apart. Each colour component (R, G, B)is scaled separately.

[0049] It should be noted that both soft transition tuning and vignettecorrection imply scaling of the pixel values by a factor that is afunction(x, y) position in the image. Even though the requirements inrespect of accuracy and coefficient range can be implemented under theuse of the same scaling function. The user (the technician for theset-up) should however regard the two functions as being separate.

[0050] Gamma Correction

[0051] Gamma correction is performed by modifying the colour values bymeans of the function C^(γ), in which c is the colour component(normalised to the range 0-1) and γ is a constant depending on thetransfer function of the projection device. The gamma correction isperformed in order to give an approximately linear relationship betweeninput values and the perceived output intensity, cf FIG. 6.

[0052] The gamma correction function also includes control of blacklevel and white point. The R, G and B components must be handledseparately in order to compensate for variable projectioncharacteristics and colour balance.

[0053] The gamma correction may be implemented as a look-up table (or aset of 3 look-up tables) for the colour components or by a pixel-wiselinear interpolation function in order to provide a perception ofconstant intensity irrespective of how the re-sampled pixel grid matchesthe input grid. The gamma correction must be adjustable.

[0054] Referring to FIG. 1, the signal processing required to providethe above functions will be described in greater detail, FIG. 1 showinga sequencer 1 that initiates a number of events based on the signalsHSYNC, VSYNC and PIXCLK from the image delivery unit. The sequencerincludes a line delay counter that counts down to 0, a frame startregister, a line counter that counts up and a status device forcontrolling the sequencing.

[0055] In the sequencer 1, the flank of the VSYNC signal initiates onenew video frame. The frame start address is set to the next free addressin the pixel storage unit 3. The pixel write control 2 is loaded withthe new address and reset. The line delay control in the sequencer 1 isloaded with the specified input-to-output delay (number of lines).

[0056] In the sequencer 1, the flank of the HSYNC signal initiates a newvideo line and transmits the start of line signal to the pixel writecontrol 2. If the line counter in the sequencer 1 is running, the startof line signal is transmitted to the read address generator 13, thecoefficient generator 5 and the scaling generator 6.

[0057] When the line delay counter in the sequencer 1 reaches zero, theline counter starts up, and the reading from the parameter storage unit7 and the flank of the output signal VSYNC is generated.

[0058] When the line counter reaches its maximum (equal to verticalresolution), the counter stops and transmits the end of frame signal tothe pixel write control 2, the read address generator 13, thecoefficient generator 5 and the scaling generator 6.

[0059] It should be noted that there may be overlap between the inputand output generation, i.e. the output of previous frames is notfinished before the next frame is initiated.

[0060] The pixel write control 2 contains an address generator that isincremented by 1 for each pixel coming in. The pixel write control 2 isloaded with a start address from the sequencer 1 at the start of eachframe, and will count up for 1024 pixel clocks after each HSYNC.

[0061] Data in the parameter storage unit 7 is coded as absolute valuesfor X, Y and RGB scaling factors at the start of every 4 lines. For each4-pixel group in the lines, delta values (dX, dY, dR, dG, dB) are stored(incremented from previous values).

[0062] In order to decode the parameter storage unit data, the storageunit 7 contains a set of accumulators that generate absolute values foreach pixel position based on the delta values. Each delta value is added4 times in order to generate 4 absolute X, Y, R, G, B vectors before anew set of delta values is read from the storage unit 7.

[0063] The read address generator 13 receives a pair of X,Y co-ordinatesfrom the parameter storage unit 7 for each pixel to be generated. Thevalues in the X and Y accumulators are combined and added in an adder 8together with the frame start address in order to give an address in thepixel storage unit 3. This address is used to look up the 4 closestpixels around the exact X,Y co-ordinate position.

[0064] The coefficient generator 5 receives fractions of the X,Yco-ordinates and generates weight coefficients for vertical andhorizontal mixers 9, 10. That is, vertical mixer weights are frac(Y) and[1-frac(Y)], and horizontal mixer weights are frac(X) and [1-frac(X)].The coefficient is delayed via a FIFO buffer (cf reference number 11 inFIG. 4) in order to ensure that coefficient data arrives synchronouslywith pixels read from the pixel storage unit 3.

[0065] The scaling generator uses R, G and B data for each pixel readfrom the parameter storage unit 7, generating coefficients to be usedfor colour scaling per pixel. This data is sent through the above FIFObuffer 11 as a mixer coefficient in order to be synchronised with pixeldata.

[0066] The gamma look-up function 12 includes storage for storing apermanent gamma look-up table. This table is used to generate anon-linear output function in order to compensate for non-linearity inthe projection device.

1. A digital correction module for video projectors, designed to givenon-linear re-imaging of an image in such a manner that it appearscorrectly on a non-ideal screen (curved screen), where a correctionmodule is arranged in each of a plurality of video projectors after acommon image source in order to create a large projected image of saidimage source, which projected image is composed of several frames,characterised i n that the correction module includes a sequencer (1), apixel write control (2), a parameter storage unit (7), a read addressgenerator (13), a coefficient generator (5), a scaling generator (6), anadder (8), a pixel storage unit (3), vertical mixers (9), a horizontalmixer (10) and a look-up function device (12), for correction ofdifferences in the boundary areas of the images of the respectiveprojectors, such as geometry correction, soft transition tuning,vignette correction and gamma correction.
 2. A correction moduleaccording to claim 1, characterised in that the geometry correction partof the correction module, which is designed to give non-linearre-imaging of the image so as to make it appear correctly on a non-idealscreen (curved screen), uses a first network of control points (acontrol network) placed no more than 4×4 pixels apart, which defines theprojection to a new grid, and that linear interpolation is carried outbetween the control points in order to derive the pixel co-ordinates foreach pixel.
 3. A correction module according to claims 1-2,characterised in that the tuning function for soft transition ispreferably performed prior to the geometry correction by there beingprovided a second network of control points for individual scaling bythe scaling generator (6) of input pixels as a function of the screenposition, where each colour component (R, G and B) is scaled separatelyand the scale factor is defined with a resolution of at least 9 bits. 4.A correction module according to claim 3, characterised in that controlpoints are arranged at a distance apart of no more than 4 pixels in thehorizontal direction and up to 4 pixels in the vertical direction.
 5. Acorrection module according to claims 1-4, characterised in that thevignette correction, which compensates for the differences in intensityas a function of the position in the projected image and compensates forvariations in the perceived intensity caused by pixels coveringdifferent areas of the screen in a projector theatre, includes thescaling generator (6), which makes use of R, G and B data for each pixelread from the parameter storage unit (7) and generates coefficients tobe used for colour scaling per pixel, and that the control points forthe vignette correction are arranged at a distance of no more than 16×16pixels apart
 6. A correction module according to claim 5, characterisedin that the parameter storage unit (7) is connected to the coefficientgenerator (5), the output of which is linked to a FIFO buffer (11), sothat data sent through said FIFO buffer (11) as mixer coefficient issynchronised with pixel data.
 7. A correction module according to claims1-6, characterised in that the gamma look-up function device (12)includes storage for storing a permanent gamma look-up table designed togenerate a non-linear output function in order to compensate fornon-linearity in the projection device.